Image capture devices, such as digital video cameras or digital still cameras, are used in different applications and environments. An image capture device should be capable of producing high quality imagery from a variety of distances relative to a target of a scene. A typical image capture device performs an auto-focus process to select a position of a lens within the image capture device that achieves a sharp focus for an image frame of the scene. An image capture device may perform an active auto-focus process, a passive auto-focus process, or a combination of the two processes.
In the case of a through-the-lens (TTL) passive auto-focus process, the auto-focus process included in an image capture device calculates a focus value, i.e., a score that represents a degree of focus, at several sampled lens positions and selects the position of the lens for the scene based on the focus values. Moving the lens to each of the sampled lens positions is independent, i.e., asynchronous, of calculating focus values for the scene. In other words, the auto-focus process calculates focus values for the scene during each frame, but has to know which one of the focus values is valid for the lens position after the lens movement.
A sensor array included in the image capture device obtains image information for the scene during each frame and the auto-focus process calculates focus values for the scene from each frame. However, in order to get a valid focus value for the scene at a lens position, the auto-focus process has to wait for a valid frame that initiated integration of image information for the scene after lens settlement at the lens position. The auto-focus process may discard the focus values calculated for the scene during the corrupted frames that initiated integration of image information before the lens has settled.
Depending on the type of sensor array, e.g., charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS), the first valid frame may be two or three frames after lens settlement. Therefore, the auto-focus process may have to wait a few frames to calculate a valid focus value for each of the sampled lens positions. The auto-focus process may determine a size and direction of movement for the lens to a next lens position only after calculating a valid focus value for the current lens position.